The present invention relates to a novel and simple method for evaluating a physical property of relatively hard, stiff materials such as plastics. More specifically, a simplified method is disclosed wherein the resistance of materials to breaking under sudden impact is measured.
Various methods and apparatus have been devised over the years for measuring the mechanical properties of relatively stiff materials. In the plastics industry it is of critical importance to both the manufacturer and the user of plastics to have at their disposal simple and reliable test methods for measuring the response of plastic materials to certain stresses such as tension, shear, bending, impact and the like. The fabricator of finished goods in particular must be able to quickly perform tests on the various raw material plastics used by him to insure that these materials meet minimum performance specifications.
Measurement of resistance to impact is of critical importance in obtaining a physical property profile of a given material, and numerous tests have been devised to perform such measurement. For example, U.S. Pat. No. 403,676, issued in 1889, teaches a pendulum type device comprising a support, a swinging hammer and a scale whereby the force required to break a mounted test bar sample can be measured. U.S. Pat. No. 1,462,813 discloses a different variation of the same idea whereby a hammer is pivotally mounted and caused to drop upon a test specimen by force of gravity from a variable height. U.S. Pat. No. 2,617,293 discloses a Charpy or Izod type of impact test as applied to notched test specimens in the shape of rods or bars, wherein the notch is cylindrical in shape and contains a snugly fitting hard cylindrical core inserted therein.
The most common tests used at present are the "Izod" and "Charpy" methods which are described in detail by The American Society for Testing and Materials under test method ASTM D-256-73. The apparatus employed in these tests consists of a cantilever beam or simple beam pendulum device. Other impact tests which are in use include the so-called tensile impact test and drop weight impact tests.
While the ASTM test methods have met with varying degrees of approval, they all involve a rather elaborate procedure in the preparation of test specimens. For example, test specimens must be of a certain geometry and must be molded in a controlled manner. This usually means that the plastics fabricator must mold or extrude a sufficient quantity of the material, often using production equipment, under precisely controlled processing conditions. This processed material must then be cut or machined into the required geometric shape. For Izod and Charpy, the test specimens must also be precisely notched by machine, which introduces a potentially major source of variation. Next the specimens must be conditioned at about 23.degree. C. and 50% RH for a period of not less than 40 hours. The drop weight impact tests suffer from the deficiency that a large number of specimens are required to obtain a numerical test value, basically because the test is a go or a no-go type, and the criteria for product failure are more difficult to define.
The value of such test methods as a quality control tool is thus somewhat diminished because often small variations in specimen preparation and conditioning from day to day and between different technicians can be a source of error as far as recorded impact resistance is concerned. Also, the large amount of time involved in preparing and conditioning specimens means that the user of the materials must wait for a period of days to verify that a given lot of plastic meets his minimum impact specifications.
Accordingly, it is an object of the present invention to provide a simple and effective method for determining impact properties of relatively stiff materials.
Another object is to provide a method for determining impact properties of plastic materials wherein the preparation of test specimens is greatly simplified.